The invention is based on a priority application EP 05292008.9 which is hereby incorporated by reference.
The invention relates to a method for amplifying a time-varying optical signal, in particular a burst-mode optical signal, and an optical amplification unit construed for implementing the method.
Burst-mode operation of optical systems will become more important with the widespread deployment of passive optical networks (PONs). The next generation of PONs is foreseen to involve optical amplification. Also, for metro and core networks there have been system proposals that rely on bursty optical signal transmission. However, amplification of such time-varying optical signals, in particular burst-mode signals, presents some difficulties. Namely, when amplifying a burst-mode optical signal in an optical amplifier such as an EDFA (erbium-doped fiber amplifier), the input power to the amplifier varies depending on such parameters as burst length, length of inter-burst gap, consecutive identical digits (CIDs), and amplitude variations between the bursts, the input power variation resulting in amplification of the signal with non-constant gain over time. Such gain variations may not only cause amplification of subsequent bursts with a different gain, but may also lead to gain variations in an individual burst, so-called transients, as described below.
FIG. 1a shows a burst-mode optical signal 1 with three consecutive bursts having identical amplitude and being applied to the input of a conventional EDFA 2. The resulting amplified signal 3 is not amplified properly, i.e. with constant gain, by the EDFA 2, as the third of the amplified bursts shows a gain transient, i.e. it starts with a higher amplitude than the preceding bursts, which decreases slowly to the amplitude of the preceding bursts (dashed line), the transient being caused by the long inter-burst gap between the second and third burst.
Likewise, FIG. 1b shows a further burst-mode optical signal 1′ with three bursts having constant inter-burst length, the third of which being a strong burst, i.e. having an amplitude which is much larger than that of the preceding bursts. After amplification in the EDFA 2, the strong burst of the amplified signal 3′ first presents the nominal amplitude which is expected for amplification with constant gain (dashed line), but afterwards shows a transient, i.e. decreases to an unwanted lower amplification level.
The unequal amplification as described in connection with FIG. 1a and FIG. 1b is particularly strong when several EDFAs 2 are concatenated, which will result in difficulties in the treatment of these signals being input to a receiver, in particular when transients are present, such that the signal gain varies over the duration of one and the same burst. Such transients require threshold adjustments of the receiver during a burst, and the large optical power spikes of the transients may even result in damaging of optical components of the receiver (photodiodes, component facets).
For keeping constant gain when amplifying time-varying signals, different solutions have been proposed in the state of the art. One such solution consists in performing gain clamping either by optical feedback (using a ring laser) or by injecting a strong idle signal of a different wavelength into the EDFA. However, gain and output power in a ring laser set-up are subject to relaxation oscillations (in the kHz range) and dissipate the major part of the energy stored in the EDFA, as is the case with an idle signal. A further solution described in the art is active control of the optical pump power level after detecting the optical input (and output) power levels, yet gain control via pump power is an indirect method that involves energy transfer dynamics that in turn depend on external parameters like pump and signal power, wavelength etc.